bpf: Add the assign_scalar_id_before_mov function

Extract the common code that generates a register ID for src_reg before
MOV if needed into a new function. This function will also be used in
a following commit.

Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240108205209.838365-7-maxtram95@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>